1. Field of the Invention
This invention concerns a catalytic combustion-type exhaust gas processing device for deodorizing and purifying noxious/smelly components contained in exhaust gases through oxidative decomposition in a series of catalyst layers, as well as a coating drying furnace that utilizes such a catalytic exhaust gas combustion device.
2. Description or the Prior Art
In those facilities such as drying furnaces for use in coating and printing, manufacturing apparatus for plastics or plywoods, food processing facilities, industrial wastes processing facilities and facilities for manufacturing fire extinguishing agents or perfumes, which use paints, inks, solvents, adhesives, synthetic resins or chemical agents, noxious/smelly components such as alcohols or esters and phenols or aldehydes which are noxious and have peculiar odors are produced tending to deteriorate the air quality of the workplace.
Therefore, it has been required to discharge such exhaust gases containing noxious/smelly components out of the facilities, but it has been prohibited to release them directly to the atmosphere, in view of the desire for prevention of pollution. Accordingly, the exhaust gases have usually been deodorized and purified by being passed through a catalytic combustion-type exhaust gas processing device and discharged after the noxious/smelly components are finally decomposed oxidatively into non-toxic and odorless gaseous carbon dioxide and water.
As the catalyst for constituting the catalyst layer in the exhaust gas processing device, various catalysts such as platinum, palladium, cobalt oxide and manganese oxide have been used. Among them, those catalysts comprising platinum group metals such as platinum (Pt), palladium (Pd) and a platinum-palladium mixture (hereinafter collectively referred to as a platinum-type catalyst) have been used generally since they have an extremely high oxidizing activity and are thermally stable.
However, although a platinum-type catalyst can promote the dehydrogenating reaction of phenols or alcohols and effectively produce aldehydes, it has been confirmed experimentally that even such platinum-type catalysts are poor in the performance of further converting the aldehydes into carboxylic acids and finally decomposing them into non-toxic and odorless, gaseous carbon dioxide and water. Accordingly, exhaust gases deodorized in an exhaust gas processing device having a catalyst layer composed only of a platinum-type catalyst contain a great amount of aldehydes as the intermediate products of the catalytic oxidation, which causes atmospheric pollution.
Further, in a drying furnace for use in coating adapted, for example, for baking or drying automobile bodies, since the inside of the furnace is usually heated to such a high temperature (to about 170.degree. to 180.degree. C.) for reacting to harden the paint films, noxious/smelly components such as organic solvents, for example, thinner and toluene, curing agents or paint resins are evaporated from the paint films. When the concentration of the noxious/smelly components in the furnace is increased, it causes yellowing and defective drying of the paint films, thereby lowering the quality of products and worsening the working circumstances. Accordingly, the exhaust gases in the furnace have also been deodorized and purified in a catalytic combustion-type exhaust gas processing device and then discharged into the atmosphere.
Since the gases deodorized and purified by passing through a catalyst layer of an exhaust gas processing device are heated to a high temperature of about 300.degree. to 500.degree. C., which is the reaction temperature of the catalyst, there has recently been proposed a technology for effectively utilizing the processed gases as a heat source for the inside of the furnace by recycling them into the furnace again without releasing them directly to the atmosphere, for example as described in Japanese Patent Publication Nos. 175785/1983 and 12946/1984.
However, since a platinum-type catalyst has been used for the catalyst layer in the exhaust gas processing device of this kind, processed gases recycled through the furnace contain a great amount of aldehydes, and it has been experimentally found by the present inventors that the concentration of oxo compounds or the like having aldehyde groups in the furnace is increased, to produce a great amount or tarry or resinous substances that cause yellowing or inter-layer defoliation of the paint films.